I. Field of the Invention
The present invention relates generally to data communication, and more specifically to a rate adaptive transmission scheme for multiple-input multiple-output (MIMO) communication systems.
II. Background
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
In a wireless communication system, data to be transmitted is typically processed (e.g., coded and modulated) to provide data symbols. For a MIMO system, one or multiple streams of data symbols may be sent from a transmitter to a receiver. Multiple data symbol streams may be transmitted in parallel from multiple transmit antennas using spatial multiplexing, which exploits the additional dimensionalities of the MIMO channel. To attain high throughput, it is desirable to transmit as many data symbol streams in parallel as possible. However, the number of data symbol streams that may be transmitted and the rates that may be used for these streams are typically dependent on the channel condition. Various transmission schemes for spatial multiplexing are currently available, including (1) an “antenna multiplexing” scheme that transmits one data symbol stream from each antenna and (2) an “eigenmode multiplexing” scheme that transmits one data symbol stream on each independent channel of the MIMO channel.
Alternatively, a single data symbol stream may be transmitted from multiple transmit antennas using transmit diversity to increase reliability of the data transmission. Diversity is achieved by the use of multiple transmit antennas as well as multiple receive antennas to provide a number of propagation paths for the data symbol stream. Transmit diversity may be used if greater reliability is desired or if the channel condition is so poor that it is better to use all of the available transmit power for one data symbol stream. Various transmission schemes for transmit diversity are currently available, including (1) a “space-time diversity” scheme described by S. M. Alamouti in a paper entitled “A Simple Transmit Diversity Technique for Wireless Communications,” IEEE JSAC, October 1998, and (2) a “delay diversity” scheme described by B. Raghothaman et al. in a paper entitled “Performance of Closed Loop Transmit Diversity with Feedback Delay,” Thirty-Fourth Asilomar Conference on Signals, Systems and Computers, 2000.
To achieve high performance, a MIMO system may be designed to support one or more transmission schemes for spatial multiplexing and one or more transmission schemes for transmit diversity. For such a MIMO system, in any given transmission interval, a specific transmission scheme may be selected for use depending on the channel condition and the desired result (e.g., higher throughput or greater reliability). However, conventional transmission schemes for spatial multiplexing are often quite different in design from conventional transmission schemes for transmit diversity. Thus, the complexity of the transmitter and receiver in the system may be greatly increased if they are required to support multiple (and different) transmission schemes for spatial multiplexing and transmit diversity. Moreover, for high performance, it is desirable to fully utilize the total transmit power available for the system and the full power available for each of the NT transmit antennas for data transmission, regardless of the number of data symbol streams being transmitted.
There is therefore a need in the art for a transmission scheme that can support spatial multiplexing, provide transmit diversity, and fully utilize the available transmit power in MIMO systems.